How Does a Motorcycle Capacitor Discharge Ignition Work?

by Chris Gilliland
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Most motorcycles built after 1980 use a Capacitor Discharge Ignition System to power their engines, replacing the older points-type mechanical contact breaker ignition. CDI ignitions provide the engine with powerful, reliable power and require very little maintenance. Understanding what the CDI is and how it works can help you weed out problems within your motorcycle's ignition system.

Capacitor Discharge Ignition Construction

A basic Capacitor Discharge Ignition system is composed of four separate components: the CDI box, pickup coil, source coil and an ignition coil. The CDI box is the heart of a motorcycle's ignition system, containing a series of diodes and semiconductors that amplify the 12-volt current supplied by the motorcycle's battery between a 400- to 600-volt range. The amplified current transfers to a capacitor, a device used to store an electrical current for short periods, housed within the CDI box. The pickup and source coils, a pair of hall-effect sensors responsible for charging and discharging the capacitor, mount near the magnetic flywheel attached to the end of the engine crankshaft. At the output side of the ignition system is the ignition coil, which amplifies the current further before releasing it into a spark plug.

Basic Operation

As the motorcycle's engine runs, a notch cut into the side of the magnetic engine flywheel passes between the source and pickup coils. The sudden drop in the magnetic field triggers the source coil, allowing a 12-volt current to flow into the CDI box. The current amplifies to a higher voltage, usually between 400- and 600-volts, and is stored within the capacitor. A drop in the magnetic field triggers the pickup coil and the capacitor discharges its stored current into the ignition coil. The current passes through a tightly wound wire within the ignition coil, called a primary coil, where it is amplified further to an excess of 10,000 volts. The final output current feeds through the ignition coil spark plug cable and into the spark plug, where it discharges within the engine's combustion chamber. This cycle repeats itself at every revolution of the engine.

Maintaining a CDI System

Under normal circumstances, a CDI-type ignition system requires very little attention aside from regular spark plug changes. Some motorcycles allow for some adjustment in the ignition timing, which changes the points at which the source and pickup coils are triggered, by using a movable coil plate. Ideally, the pickup coil should trigger the ignition coil just before the engine's piston reaches the top of its stroke. Moving the plate against the flywheel's rotational direction advances the timing and creates a spark sooner. Alternatively, moving the plate in the same direction of the flywheel retards the timing, generating the spark later. A mark along the edge of the coil plate will indicate the timing position relative to the position of the piston. Adjustments to the ignition timing are not usually required unless the motorcycle is being used for competition. Such adjustments can affect the machine's reliability unless performed carefully.

Troubleshooting A CDI ignition System

Most CDI ignition problems experienced in a street-ridden motorcycle originate within the charging system or battery. In order to generate the current needed to produce a spark, the battery must supply the CDI box with a full 12-volt current. Unfortunately, the battery must also power the motorcycle's lights as well. If your motorcycle's engine turn over, but doesn't start, completely charge the battery with an automatic battery charger before checking anything else. If the motorcycle still refuses to start, perform tests with a multimeter to determine if the CDI box incurred damage. Factory service manuals provide the testing procedures and CDI box specifications needed by your motorcycle.

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